The IRS doesn't want explanations that aren't required by the tax rules. If you attach an explanation it's very likely that no one in the IRS will look at it. If they want an explanation they will send you a letter asking for it. And as you said, you can easily prove the correctness of the expense. They are not likely to audit everything in your tax return because of one item.
So my suggestion is to e-file the tax returns and wait to see what happens. There are no guarantees, of course, but I think it will probably be fine.
